1. Field of the Invention
This invention relates generally to high-speed digital communication systems. More particularly, this invention relates to a method and apparatus for flexible allocation of overhead rate in communications systems using a block-based framing structure.
2. Description of the Related Art
The meanings of acronyms and certain terminology used herein are given in Table 1.
TABLE 1ADSLAsymmetric Digital Subscriber LineATMAsynchronous Transfer ModeDMTDiscrete Multitone ModulationFDFForward Error Correction Data FrameFECForward Error CorrectionFEQFrequency Domain EqualizerFFTFast Fourier TransformIFFTInverse Fast Fourier TransformPCMPulse Code ModulationQAMQuadrature Amplitude ModulationRSReed-SolomonSNRSignal-to-noise ratioVDSLVery High Speed Digital Subscriber LineMDFMultiplexer Data FrameSTMSynchronous Transport Module
Discrete multitone (DMT) modulation is used in many types of data communication systems, among them multicarrier, very high speed digital subscriber line (VDSL) modems, as well as asymmetric digital subscriber line (ADSL). In these systems, N tones or subcarriers are modulated by quadrature amplitude modulation (QAM) two-dimensional input frequency-domain symbols. A 2N-point inverse fast Fourier transform (IFFT) then produces a corresponding time-domain symbol, expressed as a real baseband time-domain output signal of 2N real samples in each symbol period. At the receiving side, 2N samples are extracted from the time-domain signal during each symbol period. A fast Fourier transform (FFT) is used to demodulate the signal and recover the original QAM symbols on the N tones.
For example, ADSL uses 256 or 512 tones in a downlink. ADSL standards are described in further detail, for example, in the document, ITU-T Recommendation G.992.3, entitled Series G: Transmission Systems and Media, Digital Systems and Networks; Digital Sections, which is herein incorporated by reference.
A frame is typically a set of octets or other basic units, which is constructed by multiplexing different data bearers and overhead data in a predefined manner. A frame may be transmitted in one DMT symbol, or may span several symbols. Constraints imposed by conventional framing structures hamper the maintenance of fine control of the rate of information transmission on the control channel.